<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <h5>欢迎亲的登录：</h5>

    用户名：<input type="text" id="username" /><br />
    密码：<input type="password" id="userpass" /><br />
    <input id="btnLogin" type="button" value="登录" />
    <div id="msg"></div>
</body>

</html>
<script>
let oBtnLogin = document.getElementById("btnLogin");
let oUserPass = document.getElementById("userpass"); 
let oUserName = document.getElementById("username"); 

oBtnLogin.onclick = function(){
    let xhr = new XMLHttpRequest();
    xhr.open("post","login.php",true);

    xhr.onreadystatechange = function(){
        if(xhr.readyState==4 && xhr.status==200){
           if(xhr.responseText=="success"){
               document.getElementById("msg").innerHTML = "登录成功";
           }else{
               document.getElementById("msg").innerHTML = "登录失败,用户名或者密码错误";
           }
        }
    }

    xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    
    let str = "username="+oUserName.value+"&userpass="+oUserPass.value;
    xhr.send(str);

}


</script>